The present invention relates to a cathode ray tube (CRT) display and more in detail to a video circuit for driving a type of cathode ray tubes, in which an inputted video signal is amplified by a differential amplifying circuit and applied differentially between a grid electrode and a cathode electrode of a cathode ray tube.
FIG. 6 is a circuit diagram representing a prior art video circuit for driving a cathode ray tube, in which reference numeral 1 is a CRT; 2 and 3 are grounded base type output transistors; 4 and 5 are load resistances therefor; and 6 is a well-known level shift circuit for giving the G1 (grid) electrode a negative bias, the output of the level shift circuit 6 and the output of the transistor 3 being given to the G1 electrode and the K (cathode) electrode of the CRT 1, respectively, to drive them differentially.
7 and 8 represent a pair of differential amplifying transistors; 9 is an emitter resistor for the transistor 7; 10 is a transistor serving as a current source; 11 is an emitter resistor therefor; 12 is an input terminal for a negative polarity input video signal; and 13 is a bias adjusting power supply. Further, in this figure, V.sub.DC represents a DC power supply.
As clearly seen already, this circuit is operated When a video signal is inputted to the input terminal 12, an amplified output thereof is obtained on the collector side of the output transistors 2 and 3 so that differential drive is effected between the G1 electrode and the K electrode.
A primary drawback of the prior art technique described above was that the bias adjusting power supply was necessitated in order to absorb fluctuations in characteristics of the resistors 11 and 4 as well as the transistor 10.
Further there was a secondary drawback that in a transient state at a power switch-off, when the potential of the negative polarity video signal applied to the input terminal 12 began to decrease, the potential at G1 (first grid) of the CRT 1 increased, which lowered the potential at K (cathode), and as the result an excessive beam current flowed through the CRT 1, which could burn out the fluorescent surface of the CRT 1.
The above explanation should be further complemented as follows. In the transient state at turning-off the power switch not indicated in the figure, since the operation of a well-known deflecting circuit separately disposed, although it is not indicated in the figure, is rapidly stopped, the electron beam is concentrated at the center of the fluorescent surface. As the result, the fluorescent surface could be burned out by a cooperative effect thereof with the excessive beam current described above.
As another secondary drawback there was a problem that the video output circuit indicated in FIG. 6 had a pulse response dynamic range of the differential amplifying transistors 7 and 8 narrower than that obtained by a usual grounded emitter type circuit.
As described above, the prior art technique had the drawback that it was necessary to use the bias adjusting power supply 13, whose regulation was complicated and difficult.
An object of the present invention is to provide measures for overcoming the drawbacks described above and also to overcome the secondary drawbacks of the prior art technique.